Internal curing is a lot less scary than you might think. After years of researching lightweight aggregates, testing concrete samples, authoring scientific articles, and presenting his results to the industry, Jason Weiss, the Jack and Kay Hockema Professor of Civil Engineering and director of the Pankow Materials Laboratory at Purdue University, says internal curing boils down to this: With relatively minor changes, producers can make concrete more durable and crack-resistant.

During his 14-year tenure at Purdue, Weiss has gravitated toward concrete research—controlling early-age shrinkage cracking, assessing durability, and predicting concrete service life. For more than a decade, he and his students have worked to improve concrete with internal curing to meet industry demands for durability and sustainability. The research began with minimizing the potential for early-age shrinkage cracking, then extended to plastic-shrinkage cracking.

Internal curing involves replacing some of the sand in a concrete mix with pre-wetted lightweight aggregate, such as expanded clay, shale, slate, or slag, which releases water to hydrate the cement paste—and cures concrete from the inside out. With more evenly distributed water, the cement in hardened concrete reacts to make concrete more dense, less permeable, and more crack-resistant. (See "Setting a New Standard.")

Although Weiss is often the face for the topic, he stresses the research has been a team effort, involving an army of graduate students, collaborators, sponsors, state agencies, and industry partners. “When you talk with all different segments of the industry, people bring great ideas to the table that lead to more robust solutions,” he explains.

Weiss has experience bringing people together, as a past committee chair for the Transportation Research Board (placement committee) and the ACI (Committee 123 on Research and Current Developments), and as an ACI Fellow. He has been an associate editor of the American Society of Civil Engineers’ Journal of Materials in Civil Engineering, and editor-in-chief of Materials and Structures for the International Union of Laboratories and Experts in Construction Materials, Systems and Structures (RILEM). He has received many prestigious industry and academic awards.

From lab to field

Recently, Weiss worked with the Joint Transportation Research Program, formed by Purdue and the Indiana Department of Transportation, to specify internal curing for infrastructure, where durability and low maintenance are critical. With four new bridge decks cast this year, the group has learned more about transitioning from the lab to the field, and the level of improved performance internal curing can offer.

Beyond bridge decks, Weiss sees parking decks and rapid pavement repairs as good markets for internal curing. It also has potential as a green building solution, since it works well with high volumes of supplementary cementitious materials, and for shrinkage-related problems such as curling.

The next step is bringing the technology to everyday high-performance concrete. Weiss’ group helps producers put internal curing techniques into practice with minimal disruption to their production, quality control, batching, and aggregate handling procedures.

“If someone has a mixture they would like to internally cure, all they need to do is to replace some sand with an equal volume of prewetted lightweight aggregate,” Weiss says. “The impact on strength is generally minimal, but the resistance to shrinkage-related cracking is greatly reduced.”

While internal curing may not be the answer for every application, Weiss says, “We’re talking about making good concrete better, and at the end of the day, that’s a good thing.”